No doubt you should have a backup plan and implement it soon. I can't imagine anything worse than losing years worth of digital images just because one didn't take the time to back them up.
Various methods are readily available and some work better than others depending upon how large (in mega- or gigabytes) is your collection. But here is what I do in hopes that I'm covering my bases and trying to be smart.
1) After uploading pictures to my PC from my camera's flash memory and making any adjustments to said pictures (using my photo editing software) I immediately back up those images to an external hard drive. (I also back up to an internal secondary hard drive, but you may not have this option)
2) Next I take two DVDs and burn those images to the DVDs. I keep one DVD at the computer desk and the other in a fire-resistant safe (along with other important documents, etc. - everyone should have a fire-proof safe for important documents like marriage certificates, deeds, insurance packages and backup media of pictures).
3) When the DVD that I keep at the computer desk is full I mail it to my father. This way there are copies of my pictures away from my house. Fire-resistant safes are only rated to a certain temperature for a certain time and a house fire could exceed those guides. There are other natural disasters whereby you could lose or find your safe destroyed. Having a backup off-site is a very prudent course of action. Mail that DVD to a family member or friend, preferably one that lives away from your geographical area.
So to recap: on-site storage to a backup hard drive (either internal or external). On-site storage to an optical media. Off-site storage to an optical media.
While I like the idea, as mentioned by another commentator, of using a backup web site, there is always the chance such a company could go out of business, thus leaving you in a state of limbo. And it's possible that you utilize dial-up service so online backup wouldn't be a good use of time. Otherwise it's worth looking into, but I feel is unnecessary if you regularly mail a DVD/CD to someone else.